Microfluidic devices have enabled the miniaturization of laboratory devices and allowed for a quick process of sample-to-answer with a minute amount of samples. These devices have been used to separate, sort, focus, pattern, wash, and manipulate bioparticles using ultrasound waves. Acoustophoretic microfluidic devices use bulk acoustic waves in a closed fluid cavity or surface acoustic waves generated by transducers through the bottom of the device to manipulate particles suspended in a fluid. The resonance frequency of the microchannel determines the localized acoustic radiation force. The most widely used materials for fabricating acoustophoretic devices are silicon and glass, but their manufacturing process is time-consuming and costly, meanwhile tuning the position of nodal line across the width of the fluid cavity is still a challenge. Sound-soft polymer microchannels attenuate waves, decrease pressure amplitude, and increase the rate of heat generation, but still offer low manufacturing cost, high flexibility in design and assembly, and high biocompatibility. This work focuses on microchannels made of aluminum, polydimethylsiloxane (PDMS), and hybrid aluminum-PDMS channel walls. The aluminum microchannel and hybrid aluminum-PDMS microchannel microfluidic chips were manufactured and experimentally tested for acoustic manipulation of particles

This work presents a new fabrication method for acoustic microchannels that eliminates the limitations of each sound-hard and sound-soft material in terms of cost, adjusting the position of the nodal plane, temperature rise, fragility, production cost, and disposability. Aluminum and PDMS were selected as the materials used in this work for the fabrication of hybrid aluminum-PDMS microchannels. Acoustic waves are generated by a piezoelectric transducer excited by AC electric signals. The waves reflect between the cavity walls and push particles in the fluid toward a pressure wave node or anti-node depending on the contrast factor of the particle. To model the dynamic behavior of an acoustophoretic microfluidic system, we use linear acoustic equations, where the phase factor is eliminated, and the displacement and stress tensor are determined by angular frequency and time, respectively.

The hybrid aluminum-PDMS microchannel has a superior acoustophoretic performance compared to the microchannels fully made of polymers because the Q factor is higher and the acoustic energy densities are relatively smaller in the hybrid microchannel. The Q factor is calculated by dividing the bandwidth of the fluid energy resonance curve by half of its maximum or estimated by a weighted average damping coefficient. Also, the hybrid aluminum-PDMS microchannel under the one-displacement actuation system has a good performance in collecting polystyrene particles at the nodes, but the manufacturing and assembly errors can be suppressed by employing lithography techniques for manufacturing the mold and the frame.